Celestials:
MM1 Angels- Astral Deva, Solar, Planetar. Any good alignment.
MM1
Guardinals- Avoral, Lenoal. Neutral Good.
MM1 Archons- Lantern, Hound, Trumpet. Lawful Good.
MM1
Eladrin- Bralani, Ghaele. Chaotic Good.
Gehenna is Lawful to Neutral Evil. Therefore, Eladrin, being chaotic, are unlikely to end up there, being of the Chaotic bent.
Petitioners on Gehenna are marked by greed and personal ambition. They are scheming.
Therefore, sort thru the Lawful Good and Neutral Good celestial creatures, and have him sent to Gehenna for very selfish, scheming purposes. Look into the
yugoloths, the native demons of the plane, for personality characteristics that your fallen celestial can have similar.
Perhaps a Celestial of some power and prestige, who believed he had a plan to overthrow evil better than his superiors. a little reckless, strong willed. His plan was denied, he decided to lead a force ahead anyway. he was caught by his superiors, and punished.
In Gehenna, this Celestial STILL thinks he's going to defeat his enemies. He's conspiring with demons in an attempt to defeat demons.